netdev - Re: [PATCH v3 for-next 01/13] IB/core: Use SRCU when reading client_list or device

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Date:	Tue, 12 May 2015 12:23:32 -0600
From:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:	Haggai Eran <haggaie@...lanox.com>
Cc:	Doug Ledford <dledford@...hat.com>, linux-rdma@...r.kernel.org,
	netdev@...r.kernel.org, Liran Liss <liranl@...lanox.com>,
	Guy Shapiro <guysh@...lanox.com>,
	Shachar Raindel <raindel@...lanox.com>,
	Yotam Kenneth <yotamke@...lanox.com>,
	Matan Barak <matanb@...lanox.com>
Subject: Re: [PATCH v3 for-next 01/13] IB/core: Use SRCU when reading
 client_list or device_list

On Tue, May 12, 2015 at 09:07:51AM +0300, Haggai Eran wrote:
> > I'm not sure RCU is the right way to approach this. The driver core
> > has the same basic task to perform, maybe review it's locking
> > arrangment between the device list and driver list.
> > 
> > [Actually we probably should be using the driver core here, with IB
> >  clients as device drivers, but that is way beyond scope of this..]
> 
> So, I'm not very familiar with that code, but it seems that the main
> difference is that in the core a single driver can be attached to a
> device. 

Roughly, a bus (IB would be a bus) has devices attached to it, and
devices have drivers attached to them. Bus:Device is 1:N,
Device:Drvier is 1:1. 

There a a couple of reasons to be interested in re-using the driver
core, perhaps the best is that it has all the infrastructure to let us
auto-load client modules...

> I guess a similar thing we can do is to rely on the context we associate
> with a pair of a client and a device. If such a context exist, we don't
> need to call client->add again. What do you think?

I didn't look closely, isn't this enough?

device_register:
 mutex_lock(client_mutex);
 down_write(devices_rwsem);
 list_add(device_list,dev,..);
 up_write(devices_rwsem);

 /* Caller must prevent device_register/unregister concurrancy on the
    same dev */

 foreach(client_list)
   .. client->add(dev,...) .. 

 mutex_unlock(client_mutex)

client_register:
 mutex_lock(client_mutex)
 list_add(client_list,new_client..)
 down_read(devices_rwsem);
 foreach(device_list)
   .. client->add(dev,new_client,..) ..
 up_read(devices_rwsem);
 mutex_unlock(client_mutex)

[Note, I didn't check this carefully, just intuitively seems like a
 sane starting point]

Jason
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